NITROGEN ISOTOPE TRACERS OF HIGH-TEMPERATURE FLUID-ROCK INTERACTIONS - CASE-STUDY OF THE CATALINA SCHIST, CALIFORNIA

Nitrogen isotope data for metasomatized rocks, veins, and pegmatites i n the Catalina Schist subduction zone metamorphic complex allow furthe r characterization of complex, high-P/T metasomatic processes and eval uation of the scales of isotopic equilibration and fluid transport dur ing subduction-zone metamorphism. Throughout the Catalina Schist, N re sides predominantly as NH4+ in white mica, which occurs in nearly all bulk compositions (i.e., metasedimentary, metamafic and, to a lesser e xtent, metaultramafic melange) at all grades. Within each metamorphic unit of the Catalina Schist (ranging in grade from lawsonite-albite to amphibolite facies), delta(15)N values of mica in metasomatized metam afic and metaultramafic rocks are consistent with the metasomatic addi tion of N from nearby, devolatilizing metasedimentary rocks into the i nitially N-poor mafic and ultramafic rocks. Within each unit, uniformi ty of mica delta(15)N in metasomatized rocks relative to the delta(15) N of metasedimentary rocks in the same unit implies mixing of N from n earby, heterogeneous metasedimentary sources, perhaps producing fluids with uniform delta(15)N at up to the kilometer scale. However, the tr end in delta(15)N of metasomatic mica with increasing metamorphic grad e is inconsistent with larger scale up-temperature transfer of fluid ( in this case, N-2-bearing) in the Catalina Schist paleosubduction zone ; such flow (at scales of up to tens of kilometers) has been inferred through previous oxygen isotope study. Nitrogen isotope compositions a re instead believed to have been controlled at a more local scale than the O isotope systematics, due to the more rock-dominated fluid-rock mass balance for N. The delta(15)N of muscovite in leucosomes and pegm atites in amphibolite-grade metasedimentary exposures matches that of muscovite in metasedimentary hosts, implying minimal N-isotope fractio nation during migmatization processes and possible transfer of metased imentary N-isotope signatures in silicate melts. These results illustr ate the potential of the N-isotope system to yield valuable informatio n regarding fluid-rock interactions in the crust and mantle. The data for the Catalina Schist demonstrate the ability of N isotopes to trace the transfer of sediment-derived C-O-H-S-N fluids and silicate melts, and show the expected benefit of the N-isotope system in having a dif fering fluid-rock mass balance, relative to the more commonly used sta ble isotope systems, that can yield unique constraints in quantitative models of crustal fluid processes. (C) 1997 Elsevier Science B.V.